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Subsets of 1 to 10 with a single adjacent pair

Question

Count the subsets of {1,2,…,10}\{1,2,\dots,10\} in which exactly one pair of elements are consecutive integers. For instance, {3,5,6,9}\{3,5,6,9\} qualifies because 5,65,6 is its only consecutive pair.

Short answer

235235. Glue the consecutive pair into one block, so the set is jj isolated singletons plus that block, with gaps between all pieces. Placing these j+1j+1 pieces in 1010 slots can be done in (9−jj+1)\binom{9-j}{j+1} ways, and any piece may be the block, so the count is ∑j(j+1)(9−jj+1)=9+56+105+60+5=235\sum_{j}(j+1)\binom{9-j}{j+1}=9+56+105+60+5=235.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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